Pommes Duchesse Gratin

Traditionally piped through a pastry bag into rosettes to garnish a roast, this classic French potato mixture is here transformed into a rich casserole that can be prepared in advance and is still the perfect match for a prime rib roast.

Ingredients

Special equipment: a potato ricer or a food mill fitted with medium disk

Preparation

Put oven rack in middle position and preheat oven to 400°F.

Prick each potato a few times, then bake in a shallow baking pan until tender, about 1 hour.

When potatoes are just cool enough to handle, halve and scoop out flesh. Force flesh through ricer or food mill into a bowl.

Heat cream with butter and garlic in a 3-quart heavy saucepan over moderately low heat until hot, then stir into potatoes. Beat in eggs 1 at a time with a fork until blended, then stir in salt, white pepper, and nutmeg. Spoon mixture into a buttered 2-quart (10-inch) round or oval shallow gratin or casserole dish. Bake until top is puffed and golden, 30 to 40 minutes.

Cooks' notes:
·Gratin can be prepared (but not baked) 1 day ahead and cooled completely, uncovered, then chilled, its surface covered with buttered wax paper and dish wrapped in plastic wrap. Bring to room temperature, about 1 hour, before baking.
·If you have only one oven, bake gratin while rib roast stands.